One conventional technique used in electronic eavesdropping is the employment of a carrier current transmitter to send sounds that have been picked up by a hidden microphone to a remote point over existing electrical power circuits. The power line is convenient not only as a source of power for the eavesdropping transmitter, but also as a transmission medium since it usually travels to a point outside the surveillance area where a carrier current receiver can be connected to the line and operated.
Carrier current transmitters for such an application traditionally operate at frequencies over 20 kHz and below 500 kHz. The high frequency limit of about 500 kHz is observed because the signal from the eavesdropping transmitter will radiate to some extent from the power line, and thus the surveillance device could easily be discovered using a radio operating in the standard broadcast bands if higher frequencies were used. Similarly, the frequency of 455 kHz that is commonly used as an IF in such receivers is avoided.
FM generally is employed in carrier current eavesdropping transmitters as it allows more immunity from noise interference. However, AM has been used successfully at times.
When attempting to detect the presence of carrier current eavesdropping signals on a power line, a tunable radio receiver or a broadband r.f. detector is used. Although the latter device can provide quick results, it reproduces much of the electrical noise existing on the line. Such noise can make weak signals difficult to detect and identify and if audible in the room, can be quite disturbing to occupants of that room. In addition, it has not been possible to distinguish a signal that emanates from an eavesdropping transmitter from electrical noise when using a detector operating unattended by a human operator. As such eavesdropping transmitters, in some cases, can be actuated from a remote point by the eavesdropper, it is desirable to have a detection device that can distinguish between r.f. signals and noise and that can indicate automatically (i.e., without requiring the presence of a human operator) when an r.f. signal appears on the line being monitored. It also is highly desirable that the detection device not make sounds or otherwise radiate energy that might alert the eavesdropper to the detection effort.
In summary, it is an object of the invention to provide a silently operating detection device that can automatically detect signals between 20 kHz and 500 kHz when connected to a power line in an area to be protected from electronic eavesdropping.